Pulse signaling system



ct. 31, 1944, B. TREVOR 2,361,437

PULSE SIGNALING SYSTEM Filed D80. 24, 1940 3 Sheets-Sheet 1 One iimslralzxzqedwe/ Sm. fi t Zirrze ATr O RNEY @Ct. 33, 1944 B TREVOR 2,361,437

PULSE SIGNALING SYSTEM Filed Dec. 24, 1940 3 Sheets-Sheet 2 INVENTOR wfimikewr ATTORNEY Um 3H1, WM, B. TREVOR PULSE SIGNALING SYSTEM Filed Dec. 24, 1940 3 Sheets-Sheet 3 ATTORNEY Patented Oct. 31, 1944 PULSE SIGNALING SYSTEM Bertram Trevor, River-head,

N. Y., asslgnor to Radio Corporation of America, a corporation of Delaware Application December 24, 1940, Serial No. 371,551

7 (Claims.

This'invention makes use of the fact that if a transmitter is keyed with regularly spaced short pulses having a duration of only a fraction of the total time, the peak power transmitted can be correspondingly increased without exceeding the normal dissipation rating of the tubes. This increases the efficiency of operation at the transmitter, and when used with novel receiving means arranged as I will disclose hereinafter, raises the signal-to-noise ratio at the receiver, and improves the receiver threshold level, thereby improving the reception obtainable heretofore with a given power transmitter or permits equivalent reception with a transmitter of less power.

In describing my invention, reference will be made to the attached drawings wherein:

Figs. 1 and 2 show the form of the wave energy transmitted by my system;

Fig. 3 shows the essential elements of a receiver arranged in accordance with my invention; I

Fig. illustrates diagrammatically means for generating and transmitting a signal of the form illustrated in Figs. 1 and 2; while Fig. 4 illustrates a receiver arranged in accordance with my invention including means for providing the control impulses at th receiver which synchronize the same with the transmitter.

As an example, suppose a. transmitter were to be keyed on for a time duration of 5 microseconds and off for a time duration of 50 microseconds in regular intervals then the peak power of the pulses could be made ten times the normal steady-state peak power. In Fig.1 is shown a typical example of the output of a transmitter 01 this nature. The transmitter is on a small fraction of the transmittin time. The rate of keying off and on of the transmitter is above audibility. Each of the impulses are modulated as for example by wide band frequency modulation. The nature of the impulses, so modulated, are shown in Fig. 2.

To make best use of this type of transmission, it will be assumed that a receiver is used having the elements shown by block diagram in Fig. 3. This receiver comprises a wave pick-up means such as an antenna ill, feeding a radio-frequency amplifier l2. which in turn feeds a demodulator of the heterodyne type including a detector I 4 and oscillator H6. The difierence frequency is fed from the detector it to the intermediatefrequency amplifier I8 and from this amplifier to a limiter 20 wherein the amplitude variations 'of the frequency modulated pulses are removed. The limited pulses are fed to a frequency modulation detector means in 22 which includes frequency discriminating circuits per se well known in the art. The demodulated output is supplied to a modulation amplifier 24 and thence to an output means 26 and to a unit 28 wherein controlling voltages are produced. This means 28 includes a wave generator, wave former, etc., which, under control of the demodulated frequency modulated impulses (Fig. 2) provides a potential on a lead connected to l8 and 20. This potential acts on receiver stages such as [8 and 20 to cause the same to amplify and pass freely the said impulses, and to become inoperative for this purpose between the said pulses so that noise on the received wave as transmitted or as added thereto in the receiver stages is prevented from reaching the demodulator in 22.

The pulse generator 28 output is synchronized by the transmitted pulses. The output of the receiver pulse generator serves to key the intermediate-frequency amplifier l8 and limiter 20 on and off in such'a way that the receiver is completely inoperative during the time between.

transmitted pulses and is in normal operation during the period of each transmitter pulse. It is, of course, necessary that the pulse frequency be substantially above the highest useful modulating frequency in the output of the receiver.

The radio-frequency amplifier l2, detector l4, oscillator I 6, amplifier 24, and output 26 may be of any appropriate type as known in the art, and since these units per se form no part of the present invention, they will not be described here. It will be noted, however, that radio-frequency amplification alone may be used in place of the heterodyned amplifying and detecting means M and [6 in which case appropriate modification in the tuning of amplifier I8, limiter 20, discriminator 22, etc., will be made.

Moreover, the means for deriving the demodulation components for controlling the means producing the synchronized pulses in 28 may be the demodulator 2A supplying the output to 26 or may be a separate demodulator. In Fig. 4, I have shown the essential details of the novel means of my invention for providing from the received frequency modulated wave impulses, controlling potentials, the wave generator controlled by said potentials, and the manner in which the stages in the intermediate-frequency amplifier and limiter 20 are controlled to render the receiver inoperative between transmitted pulses and operative during transmitted pulses.

In Fig. 4, theimpulse energy is supplied say from unit M of Fig. 3 or similar means to input terminals 36 and impressed by a transformer 38 to the input grid and cathode 42 of an amplifier tube 44. The amplified frequency modulated impulses are supplied from the output of tube 44 to a second and similar stage comprising a tube 44' havin its grid 40' coupled to the secondary winding of transformer 38' and its anode coupled to the primary winding of a transformer 46 the second transformer 46 being coupled to the control grid 48 and cathode 50 of a limiter tube 52. The limiter tube 52 operates in a known manner to remove amplitude variations from the frequency modulated impulses and thelimited amplified energy is supplied to a frequency discriminator circuit 60.

This discriminator circuit is in some respects similar to that shown in Mountjoys United States application #345,913, filed July 17, 1940, Patent No. 2,280,536, issued April 21, 1942, and will-be described but briefly here. The discrim- -inator comprises two tuned circuits 62 and 64,

one of which is tuned to a frequency slightly above the mean frequency of the frequency modulated impulses and the other of which is tuned to a frequency slightly below the mean frequency of the frequency modulated impulses. The frequency variations of the impulses are converted to amplitude variations (due to the sloping characteristics of the circuits). The amplitude variations are rectified in diode rectifiers 66 and 68 to produce differential potentials in the resistances 6! and 69. These differential potentials are supplied from the outputs of'said diode rectifiers to an audio frequency amplifier 24' which may be included in the unit 24 of Fig. 3.

The operation of the limiter 52 and of the discriminator circuit including coils 62 and 64 and diode rectifiers 66 and 68 is well known in the art and needs no further description here.

The frequency modulated wave energy impulses are also supplied from the secondary winding of transformer 46 to the anode I0 and cathode 12 of a diode rectifier having in series therewith a potentiometer resistance I4 and a resistance IS, The demodulated impulses produce potentials in the potentiometer resistance I4 which are supplied between the grid I6 and cathode 11 of an impulse amplifier tube I8 by way of a second potentiometer resistance I5. The anode 80 of this tube is connected as shown to an output impedance 82 shunted by a filtering condenser 84. Condenser I9 is also provided to bypass radio frequency at the grid I6 of tube I8. The potentials supplied to the grid I6 are negative and produce positive impulses on the anode of tube I8. These positive potentials are supplied by way of leads 88 and 86 to the control grids 40 and 40' of tubes 44 and 44', respectively, and being positive raise the potential on the said grids to increase the gain of these tubes. Additional stages may be controlled by potentials supplied from lead 89 to control electrodes in tubes thereof in a similar manner.

' The positive pulses from the resistor 82 are fed to the intermediate-frequency amplifier control grids in synchronism with the application of received frequency modulated impulses thereon so that for the duration of each received pulse, the intermediate-frequency amplifier gain is raised to a high value. By adjusting the potential supplied to the grid I6 b means of potentiometer I4 and I5, connected as shown, the ain of the intermediate-frequency amplifier may be made low during the space between pulses, thus avoiding noise reception during this time.

' quency modulation type having, in addition, the

in Crosby's United States application Ser. No.

pulse keyer described herein. The transmitter used herein may be in many respects as disclosed 136,578 filed April 13, 1937, Patent No. 2,279,- 659, issued April 14, 1942, and in Crosby's United States application Ser. No. 358,385 filed September 26, 1940 with modifications in accordance with my invention.

In Fig. 5, I have shown a transmitter satisfactory for the production of frequency modulated waves in accordance with my invention. In Fig. 5, I00 is a wave generator of the electron discharge tube type having its anode coupled as shown to the desired amplifiers and/or frequency multipliers comprising cascaded stages I02 and I04 which, in

turn, may be coupled to additional stages and any other output arrangement desired. The oscillator I00 includes a tuned tank circuit I06 connected between its anode and control grid and operating regeneratively for the production of oscillations to be frequency modulated and transmitted.

The frequency of the produced oscillations are modulated by modulating potentials supplied to the primary winding of a transformer I08, the secondary winding of which is connected between the grid H0 and cathode II2 of a reactance tube II4. Reactance tubes are well known in the art and this tube may be, in general, as disclosed in the said Crosby applications and. other applications. The anode II6 of this tube is connected to a point on the tank circuit I06 to derive therefrom a, radio-frequency potential ofa predeterminedphase. A phase shifting circuit C, R, R, and the inherent grid-to-cathode capacitance of grid I I8 and cathode I I2, supplies radio-frequency 40 voltage to grid H8 in this tube substantially in phase quadrature with the anode voltage to produce in the tube a reactive effect which supplements the reactance of the tank circuit I06. This reactive effect is inductive or capacitive, depend- I ing on whether the current in the tube to the plate leads or lags the plate voltage. In the modification illustrated, the current leads the plate voltage and the reactive effect is complex but may be considered capacitive. Modulation of the tube current modulates the said capacitive effect thereby modulating the reactance in the tank circuit and the frequency of the oscillations generated in accordance with the modulating potentials supplied to the grid H0.

Radio-frequency voltage is also supplied from tank circuit I06 by inductance I20 to the tuned circuit I24 connected with the grid I26 to a mixer tube I28 also supplied at its grid I30 with oscillations of substantially constant but different frequency from a source I32. The oscillations from tank circuit I06 and source I32 are mixed in tube I28 and a beat frequency is supplied to a frequency discriminator circuit I34 similar in many respects to the discriminator circuit 60 of Fig. 4. In this discriminator circuit the frequency variations or drifts of mean frequency of the 0scillator I00 are converted to amplitude variations, rectified in diodes I38 to cause corresponding currents to fiow in resistance I31 and I39 to produce therein potential variations, the difference of which is supplied by lead I40 to the control grid H8 of reactance tube II4 to additionally control the reactance which tube II4 adds to tank I06 and this control is in the direction to counteract any tendency .of the oscillator I00 to deviate in frequency of operation from its assigned mean frequency. The resistance R2 and condenser CI are of a value to filter out all variations in potential of a frequency greater than the lower modulation potential frequency.

In order to monitor the wave generator, wave stabilizing and wavelength modulating action of the reactance tube controlled oscillator, I connect the grid G of an amplifier and coupling tube T through a coupling condenser CC to the lead I40, and couple the cathode K of this tube to the anode end of resistance I39. The output of this tube is supplied to a meter or other potential and/or current variation indicating means through a transformer TI.

The frequency stabilized and frequency modulated oscillations produced in the means described above are as stated hereinbefore, amplified and/or frequency multiplied in stages I02 and IM for transmission purposes. In accordance with my invention, I interrupt this frequency modulated wave transmission as illustrated in Figs. 1 and 2 and in Fig. 5 I have illustrated one means for doing this. The square wave form generator I 20 of any desired type such as, for example, the type used for testing television video equipment is used to provide waves of square wave form and supply the same byway of a network comprising a variable condenser C2 and resistance R to a diode 92b having in its output a resistance I 28. The rectified potentials from resistance I28 (of the frequency of the square wave) are supplied to the control grid lid of a wave amplitude limiting tube I32 and from the anode I36 of this tube to the control grid I36 of a wave amplitude limiting tube I38 having its anode M0 connected to an adjustable resistance M2. The resistance I 42 is connected to the control grid Mt of a frequency multiplier and/0r amplifier stage comprising tube Illd. This output may be supplied to additional stages, if desired.

Adjustable condenser C2 and diode bias resistor R permits adjustment of the effectiveness of the square wave from I20 on diode I26 to thereby permit adjustment of the time duration of the pulses supplied by the rectifier to the grid I36 of the next stage amplifying these pulses. That is, by the use of condenser C2 and resistance R, the percent of mark time and space time is adjusted as desired. These impulses are impressed on the grid I3t for limiting in tube I32 and then impressed on the grid I36 for limiting in tube I 38 to give desirable fiat-top form. The output impulses by means of I 42 are adjusted in amplitude to a value such that tube MB is biased to be inactive the desired time duration and active the remaining transmission time to produce an output as illustrated in Figs. 1 and 2.

With the transmitter and receiver in operation, it will be observed that reception occurs only during the pulse time and no reception of signal or noise occurs during the interval between pulses. In this way, the peak power of the transmitter can be increased as the percent mark of the pulses is decreased without exceeding the normal dissipation of the tubes.

As an example, suppose we have a frequencymodulation transmitter operating at some ultrahigh frequency having a maximum deviation frequency of +90 kc. and a maximum modulation frequency of kc. Under these conditions, the normal receiver band width would be 200 kc. Suppose that the transmitter pulses occur every 100 microseconds and each pulse is 10 micro- III 200 kc. is then required to accept these pulses and a receiver band width of 200+200=400 kc. is required to accept the pulses composed of the above frequency-modulated carrier. It will now be observed that the transmitter is on the air only of the total time so that its peak power may be raised to ten times normal which is an increase of 10 db. This gives a signal-to-noise ratio 10 db. greater than that obtained with the original wide band frequency-modulated signal when working above the threshold of the receiver. Since the, intermediate-frequency band has been increased in the ratio 400/200l=2l=3 db., the threshold has been lowered (improved).by

400 kc. band using pulse transmission, we have seconds in duration. A receiver band width of gained 1 )6.5=3.5 db. improvement in signal-tonoise ratio above the threshold and we have improved the threshold by 7+3=l0 db.

The advantages for this system are that a small transmitter may be used to give an equivalent power output many times its normal rating, and, at the same time, the threshold at the receiver has been lowered by a very substantial amount which means greater service range.

What is claimed is:

1. In a signal modulation system, means for producing frequency modulated wave energy, means for interrupting said frequency modulated wave energy periodically to produce pulses which are short compared to the time interval between them, a receiver for the short pulses of frequency modulated wave energy including a plurality of stages, and means including a rectifier coupled to one of said stages and back-coupled to a. preceding stage for opening the receiver in synchronism with said interruptions to thereby render the receiver operative substantially solely during the times the pulses arrive at said receiver.

2. In a receiver adapted to the reception of wave length modulated wave energy transmission which is interrupted periodically at a rate above the highest modulation frequency rate, a wave receiving circuit excited by said interrupted modulated wave, an amplifying circuit coupled to said receiving circuit, an amplitude limiting circuit coupled to said amplifying circuit, a frequency modulation detecting circuit coupled with said limiting circuit, a rectifier for deriving potentials the amplitude of which increase between interruptions of the received wave, and a control circuit responsive to the derived potentials for controlling the gain of the amplifying means of said recleiver in accordance with said derived potenre. s.

3. In a receiver for the reception of frequency modulated wave energy transmission which is interrupted periodically at a rate above the highest modulation frequency rate to produce modulated wave impulses, wave impulse receiving and amplifying circuits, frequency modulation detecting circuits coupled with said amplifying circuits, a rectifier excited by amplified impulses for deriving potentials the amplitude of which decrease during the interruptions of the received wave, and

a control circuit controlled by the produced potentials'for reducing the gain of the amplifying circuits of said receiver during said interruptions in the modulated wave.

4. In a wave length modulation system, wave generating, wave mean frequency stabilizing and wave length modulating means, transmitting means coupled with said aforesaid means, means for interrupting the transmission from said system periodically for equal length intervals and at a rate above the highest modulation frequency, wave receiving and amplifying means, a wave length demodulator coupled with said amplifying means and means for changing the gain of the amplifying means periodically at those intervals between received pulses and at a rate equal to the interruption rate at the transmitter and in synchronism therewith, said last means including a rectifier coupled between the output and input of said amplifying means.

5. In a receiver adapted to the reception of modulated wave energy impulse transmission wherein the impulses are short compared to the time interval between them and the impulse rate is above the highest modulation frequency, wave receiving and amplifying means biased to a critical value for reducing noise, modulation detecting means coupled with said amplifying means, and means for increasing the gain of the amplifying means of said receiver periodically above said critical value at a rate equal to the impulse rate of transmission and at those times during which the pulses are being received, including a rectifier deriving energy from said receiver and back-coupled to the input of said amplifying means.

6. In a receiver adapted to the reception of modulated wave energy transmission which is interrupted periodically at a rate above the highest modulation frequency rate, a. wave receiving circuit excited by said interrupted modulated wave energy, an amplifying circuit coupled to said receivingcircuit, a modulated detecting circuit coupled to said amplifying circuit, a rectifier for deriving potentials the amplitude of which increase between interruptions of the received wave energy, and a control circuit responsive to the derived potentials for controlling the gain of the amplifying means of said receiver in accordance with said derived potentials.

7. In a receiver for the reception of modulated wave energy transmission which is interrupted periodically at a rate above the highest modulation frequency rate to produce modulated wave impulses, wave impulse receiving and amplifying circuits, modulation detecting circuits coupled with said amplifying circuits, a rectifier excited by amplified impulses for deriving potentials the amplitude of which decrease during the interruptions of the received wave, and a control circuit controlled by the produced potentials for reducing the gain of the amplifying circuits of said receiver during said interruptions in the modulated wave received.

8. A communications system comprising means for producing radio frequency energy and for varying the instantaneous frequency of said energy, and means for transmitting the modulated energy in the form of short pulses whose peak power greatly exceeds the normal steady-state value of a continuous signal, in combination with receiving means responsive substantially only during time periods when pulses are received.

BERTRAM TREVOR. 

